1. Field of the Invention
The present invention relates to electronic devices and fabrication methods thereof, and more particularly, to an electronic device and a fabrication method thereof for facilitating reworking.
2. Description of Related Art
Generally, semiconductor packages, such as CPUs and memories, are disposed on printed circuit boards of electronic products (such as computers) and the printed circuit boards serve as communication interfaces for various kinds of electronic elements, such as I/O devices, multimedia and communication devices, thereby connecting the semiconductor packages to external devices so as to form a complete electronic system.
In the development of semiconductor technologies, various package types have been developed for semiconductor products. For example, BGA (ball grid array) semiconductor packages that have reduced IC areas, high density and multiple connections have become the mainstream in the market of semiconductor packages. Further, to obtain much lighter, thinner, shorter and smaller semiconductor packages, chip scale packages (CSP) are developed, which are characterized in that the package size is equal to or slightly larger than the size of a chip.
Conventionally, an underfilling process is performed to bond a BGA type chip scale package to a circuit board. However, voids, overflow or insufficient underfilling easily occurs during such an underfilling process. Further, since the underfilling material is made of a thermosetting polymer, when it is cured after being heated, it becomes difficult to rework. Therefore, if a defect occurs to the package or the underfilling material, the entire circuit board must be discarded.
To facilitate reworking, Taiwanese Patent No. I251,927 discloses a detachable base. A package can be disposed on the detachable base and then the detachable base with the package can be disposed on a circuit board. If a defect occurs to the package, the detachable base as well as the package can be detached and replaced with good ones, thereby eliminating the need to discard the entire circuit board.
However, the detachable base occupies spaces on the circuit board, incurring an increase of the area and height of the circuit board and hindering miniaturization of electronic products. Further, to facilitate detachment of the detachable base, the detachable base has a limited number of pins, thus limiting the increase of the number of connection pins between the circuit board and the package and making it difficult to meet the multi-function requirement of electronic products. In addition, the use of the detachable base incurs a high cost.
In view of the above-described drawbacks, FIG. 4A to FIG. 4C show a reworking method disclosed in US Patent Application Publication No. 20090184407. Through such a reworking method, a defective package or a defective underfilling material can be replaced without the need to discard the entire circuit board or the need of a detachable base.
Referring to FIG. 4A, a semiconductor package 11 is flip-chip disposed on a circuit board 10 through a plurality of first solder balls 12 and an underfilling material 13 is formed between the semiconductor package 11 and the circuit board 10 for encapsulating the solder balls 12. If a defect occurs to the semiconductor package 11, the semiconductor package 11 can be removed by grinding.
Referring to FIG. 4B, when the semiconductor package 11 is removed, upper portions of the underfilling material 13 and the first solder balls 12 are also removed (or the underfilling material 13 and the first solder balls 12 can be completely removed) and the remaining lower portions of the first solder balls 12 serve as electrical contacts 12′.
Referring to FIG. 4C, another semiconductor package 11′ is flip-chip disposed on the electrical contacts 12′ through a plurality of second solder balls 14 and an underfilling material 15 is formed to encapsulate the second solder balls 14.
In the above-described reworking method, a mechanical grinding process is performed to remove and replace the semiconductor package 11 so as to eliminate the need to discard the entire circuit board 10 and meet the miniaturization and multi-function requirements of electronic products.
However, the grinding process is quite complicated and therefore is costly and time-consuming. Further, the grinding depth is difficult to control, which easily leads to an uneven surface of the underfilling material 13′ and a poor coplanarity between the electrical contacts 12′, thus causing a tilted bonding of the semiconductor package 11′ to the circuit board 10 and even causing an offset of the joints between the semiconductor package 11′ and the circuit board 10.
Furthermore, an error due to the grinding accuracy may cause damage of circuits or copper pads on the circuit board 10.
Moreover, the grinding process produces a lot of particles that may pollute other elements (not shown) on the circuit board 10.
In addition, although an underfilling material having a low viscosity and a high CTE (coefficient of thermal expansion) facilitates removal of the semiconductor package, its low viscosity cannot meet the reliability requirement of the package assembly. Therefore, such an underfilling material cannot be used.
Therefore, there is a need to provide an electronic device and a fabrication method thereof so as to overcome the above-described drawbacks.